The present invention relates to an apparatus for compressing a moving web with an endless flexible belt wrapped around a rotatable drum, and particularly to an apparatus and technique of the nature wherein the web is compressed by the belt and roll nips while the web is guided about the heated cylindrical surface of the drum.
Presses are used to consolidate paper and panel products. Examples of this consolidation are the formation of a pulp mat from a pulp slurry, the formation of paper from wood pulp or other fibrous material, or the formation of a panel product from wood particles or flakes. Compressive forces act on and consolidate the material as it passes through the nip formed by a pair of rolls. The greater the compressive force the greater the consolidation.
The compressive forces at the nip perform another function in the formation of paper--the removal of water from the web.
The compressive force acting on a web in the nip between the two rolls is of short duration. The time that the compressive force may act on the web may be extended by the use of a belt press. In a belt press a belt is wrapped around a section of the periphery of a drum and exerts a compressive force on a web passing between the belt and drum. Tension in the belt is translated into a compressive force on the web and drum. Belt presses are used both for paper and for panel products. Gottwald et al., U.S. Pat. Nos. 3,110,612 and 3,354,035 and Haigh, U.S. Pat. No. 3,319,352 are exemplary of belt presses for paper. Gersbeck et al., U.S.Pat No. 3,891,376, Brinkmann et al., 3,938,927 and Gerhardt et al., U.S. Pat. No. 4,457,683 are exemplary of belt presses for panel products.
In my earlier U.S. Pat. No. 4,710,271, I analyzed the forces exerted on a web or mat being pressed by several of these prior art devices. The belt and drum press disclosed in my aforementioned patent represents an improvement in several ways over the earlier known devices. In particular, the prior invention increases the amount of the drum circumference made available for pressing and heating, it creates an increased magnitude of compressive force on the drum produced by belt tension, it provides for an absence of bending moment on the drum and on idler nip rolls, and no significant belt tensioning forces are transmitted to the supporting frame of the machine. The reduced stresses in the drum facilitates the use of novel improved heated drums in the press. Such drums, featuring high thermal conductivity, are disclosed in the earlier noted U.S. Pat. No. 4,710,271. This patent is hereby incorporated by reference.
In the versions of the belt and drum press disclosed in U.S. Pat. No. 4,710,271 and in application Ser. No. 094,137, the roll nip forces on the drum are produced and determined solely by belt tension and belt tensioning forces. Supplemental loading of nip rolls to increase the nip forces of nip rolls on the drum beyond those produced by belt tension is known in the art as in Haigh U.S. Pat. Nos. 3,319,352 Haigh, and Miller U.S. Pat. No. 4,740,305. However, those disclosures are inadequate to describe the particular methods of application of supplemental loading that are required in the present invention to retain the advantages inherent in the present invention.